Animals, such as mammals and birds are often susceptible to parasite infestations. These parasites may be ectoparasites, such as fleas, ticks, mosquitoes, mites, flies, sand flies and lice and endoparasites such as roundworms, hookworms, flukes and heartworms. Domesticated animals including farm animals are particularly susceptible to parasite infections and infestations.
Parasitic diseases may be caused by either endoparasites or ectoparasites. Endoparasites refer to those parasites living inside the body of the host (such as in the stomach, lungs, heart, intestines, etc.). Ectoparasites are those parasites that live on the outer surface of the host but still draw nutrients from the host. Endoparasitic infections may further be subdivided based on class of parasite involved in the infection. These parasitic infections and infestations are often associated with illness and death or reduced productivity.
Examples of endoparasites which infect animals include but are not limited to gastro-intestinal parasites of the genera Ancylostoma, Necator, Ascaris, Strongyloides, Trichinella, Capillaria, Toxocara, Toxascaris, Trichuris, Enterobius, Haemonchus, Trichostrongylus, Ostertagia, Cooperia, Oesophagostomum, Bunostomum, Strongylus, Cyathostomum and Parascaris among others, and those that are found in the blood vessels or other tissues and organs include Wuchereria, Brugia, Onchocerca, Dirofilaria and the extra-intestinal stages of Strongyloides, Toxocara and Trichinella. 
Ectoparasites which infest man and domestic animals include arthropods, such as ticks, fleas, mites, mosquitoes, lice, and the like and infestations by these parasites can result in transmission of pathogens that can cause serious and even fatal diseases.
Infestations by ectoparasitic arthropods including but not limited to ticks, mites, lice, stable flies, horn flies, blowflies, face flies, fleas, mosquitoes and the like are also a serious problem. Infestation by these parasites may result not only in loss of blood and skin lesions, but also can interfere with normal eating habits thus causing weight loss. Ectoparasitic infestations of a host can also result in transmission of pathogenic agents that cause serious diseases including but not limited to encephalitis, anaplasmosis, Babesiosis, Rocky Mountain spotted fever, Lyme disease, ehrlichiosis, West Nile virus, swine pox, malaria, yellow fever, and others, many of which can be fatal to the host. Animals may be infected by several species of parasites at the same time since infection by one parasite may weaken the animal and make it more susceptible to infection by a second species of parasites.
A parasite which is prevalent among domesticated animals is Stomoxys calcitrans (stable fly). Stable flies will feed on blood from practically any warm blooded animal, including humans, pets, and livestock. During periods of high stable fly activity, humans can be severely annoyed and this insect has been called the biting house fly. Individual flies may feed more than once per day. Peaks of feeding activity commonly occur during the early morning and again in the late afternoon. Stable flies prefer feeding on lower parts of the hosts such as the legs.
Both male and female stable flies feed on blood. The female requires blood meals to produce viable eggs. Females deposit their eggs in a variety of decaying animal and plant wastes. Stable flies breed in soggy hay, grasses or feed, piles of moist fermenting weed or grass cuttings, spilled green chop, peanut litter, and seaweed deposits among beaches, in soiled straw bedding and sometimes in hay ring feeding sites. Each female fly may lay 500-600 eggs in 4 separate batches. Eggs hatch in 2-5 days into larvae which feed and mature in 14-26 days. The average life cycle is 28 days, ranging from 22-58 days, depending on the weather conditions.
Farm animals can be severely affected by the stable fly. Since the fly takes blood meals, animals are weakened from blood loss and continual irritation. Animals such as swine, cattle, and horses infested with stable flies may show reduced weight gains, among other harmful effects. While one stable fly does not cause significant damage, 50-100 of these blood-sucking pests together with 500 horn flies cause a significant loss of blood. This can result in a significant loss of milk and beef production from animals each year.
Another important parasite is the tick genus Rhipicephalus, especially those of the species microplus (cattle tick), decoloratus and annulatus. Ticks, such as Rhipicephalus microplus, are particularly difficult to control because they live in the pasture where the farm animals graze. Other important parasites of cattle and sheep are listed as follows: myiases such as Dermatobia hominis (known as Berne in Brazil) and Cochliomyia hominivorax (greenbottle); sheep myiases such as Lucilia sericata, Lucilia cuprina (known as blowfly strike in Australia, New Zealand and South Africa). These are flies whose larva constitute the animal parasite; flies proper, namely those whose adult constitutes the parasite, such as Haematobia irritans (horn fly); lice such as Linognathus vitulorum, etc.; and mites such as Sarcoptes scabiei and Psoroptes ovis. 
More prevalent among domesticated animals, such as cats and dogs, are the following ectoparasites: cat and dog fleas (Ctenocephalides felis, C. canis. and the like), ticks (Rhipicephalus spp., Ixodes spp., Dermacentor spp., Amblyoma spp. and the like), and mites (Demodex spp., Sarcoptes spp., Cheyletiella spp., Otodectes spp. and the like), lice (Trichodectes spp., Lignonathus spp., and the like), mosquitoes (Aedes spp., Culex spp., Anopheles spp., and the like) and flies (Hematobia spp., Musca spp., Stomoxys spp., Dermatobia sp., Cochliomyia spp., and the like).
Fleas are a particular problem because not only do they adversely affect the health of the animal or human, but they also cause a great deal of psychological stress. Moreover, fleas are also vectors of pathogenic agents in animals, such as dog tapeworm (Thpylidium caninum), and humans.
Similarly, ticks are also harmful to the physical and psychological health of the animal. However, the most serious problem associated with ticks is that they are the vector of pathogenic agents, agents which cause diseases in both humans and animals. Major diseases which are caused by ticks include borrelioses (Lyme disease caused by Borrelia burgdorferi), Babesioses (or piroplasmoses caused by Babesia sp.) and rickettsioses (also known as Rocky Mountain spotted fever). Ticks also release toxins which cause inflammation or paralysis in the host. Occasionally, these toxins are fatal to the host.
Moreover, mites and lice are particularly difficult to combat since there are very few active substances which act on these parasites and they require frequent treatment.
The above list is not exhaustive and other ectoparasites are well known in the art to be harmful to animals and humans.
While it is known in the art that it is sometimes possible to combine various parasiticides in order to broaden the parasiticidal spectrum, it is not possible to predict, a priori, with any reasonable certainty which combinations will work for a particular animal or disease state. For this reason, the results of various combinations are not always successful and there is a need in the art for more effective formulations which may be easily administered to the animal and have the required solvency, stability and bioavailability.
Formulations comprising different actives are especially difficult to successfully formulate because of the challenges in achieving the required solvency, stability and bioavailability. Thus, there is a need in the art for combination antiparasitic formulations that meet the required solvency, stability and bioavailability of the parasiticides to be formulated therein.